definitions.mk: Don't use GCC '@' parameter

This parameter (which causes GCC to pull additional command line
parameters out of a file) is incompatible with ccache.

- With ccache 2.x, ccache will ignore this parameters, and potentially
compute invalid command line hashes since it is not reading the
parameter file

- With ccache 3.x, ccache will refuse to cache the files and error
with 'Unsupported compiler option'.

We still use the parameter file, but Make expands it instead.

Change-Id: I070c3877cbe2d058e1cf4754bac535e7f3498861
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>